N′-phenyl-N-alkylphenylenediamines have utility as antioxidants for lubricant oils, rubber compositions and antiozonants. U.S. Pat. No. 5,232,614 discloses the use of N,N′-dialkylsubstituted p-phenylenediamines as antioxidants in lubricating oils.
The Friedel-Crafts alkylation of N′-phenyl-N-phenylenediamine compounds using alpha-olefins and a Lewis acid catalyst such as aluminum chloride provides little or no yield of alkylated product. Instead, an aluminum alkyl catalyst and high pressure may be used to alkylate N′-phenyl-N-phenylenediamines.
The use of aluminum alkyls to alkylate a N′-phenyl-N-alkylphenylenediamine has at least two disadvantages: the compounds are not recyclable, and they may spontaneously ignite in air. Moreover, the reaction typically alkylates the non-substituted phenyl ring. Thus, for example, the use of triethylaluminum chloride, heat and pressure to alkylate N′-phenyl-N-isohexylphenylenediamine with 1-decene produces predominantly N′-2-methylnonylphenyl-N-isohexylphenylenediamine.
An ionic liquid consists of inorganic and/or organic cations and anions, and typically has a very low vapor pressure, a wide liquid temperature range, and is non-flammable. Ionic liquids can act as a catalyst and/or solvent, and have been studied for utility as solvents, electrolytes, in separations and in fluid applications such as lubricants. See Holbrey, “Industrial Applications of Ionic Liquids,” Chemistry Today 35 (June 2004); Parkinson, “Ionic Liquids Make an Environmental Splash,” 100 Chemical Engineering Progress 7 (September 2004); and Drake et al., “Structural Effects on the Physical Properties of Ionic Liquids,” Air Force Research Laboratory Report No. AFRL-PR-ED-VG-2003-12 (May 2003).
The use of ionic liquids in Friedel-Crafts alkylation has been discussed in Wilkes, Friedel-Crafts Reactions in Chloroaluminate Molten Salts,” Molten Salt Chemistry: An Introduction and Selected Applications 405 (Mamantov and Marassi Eds. 1987) and Earle et al, “Organic Synthesis,” Ionic Liquids in Synthesis 174 (Wasserschied & Welton Eds. 2003). However, neither study is directed to an alkylation reaction which permits further substitution of the phenylene ring of an N′-phenyl-N-alkylphenylenediamine compound.
An object of the invention is to provide a synthesis which permits alkylation of the phenylene ring of an N′-phenyl-N-phenylenediamine compound.
A feature of the invention is the use of an ionic liquid as a solvent and catalyst for the alkylation reaction.
An advantage of the invention is that use of an ionic liquid typically permits convenient separation of N′-phenyl-N-alkyl(alkylphenylene)diamine from the reaction mixture.